The original metal bodied router plane most people think of is the Stanley #71 followed by the Stanley #71-1/2. Of course given that the Stanley numbering system often doesn’t make sense to anyone, except for the original Stanley designers and engineers themselves, it would make sense that we might not know essentially they’re both the same plane just with a slight difference.

The easiest way to explain the difference between the two is just to look at the modern versions available at Lie-Nielsen or Veritas. Lie-Nielsen’s version is equivalent to the original #71 while Veritas has based their’s on the #71-1/2.